Sum value from an Array in JavaScript

Question

I am trying to do a addition method in a javascript variable, below is my code:

<html>
<body>
  <p id="demo"></p>
  <p id="demo1"></p>
  <p id="demo2"></p>
  <script>
  var juice = [];
  var water = [];
  var fruits = [2, 5, 7, 10,15, 25,28,34,38,45,49,52,55,57,59];
  for(var i =0;i < fruits.length;i++){
        var today = new Date();
        var numberOfDaysToAdd = fruits[i] ;
        today.setDate(today.getDate() + numberOfDaysToAdd); 
        var dd = today.getDate();
        var mm = today.getMonth()+1; //January is 0!
        var yyyy = today.getFullYear();
        if(dd<10) {
           dd='0'+dd
        } 
        if(mm<10) {
           mm='0'+mm
        } 
        today = mm+'/'+dd+'/'+yyyy+' $ ';

     juice[i] = today;
    for( var j=0; j<=juice.length; j++)
   {
        water[j] += juice[j][4];

   }

  }
  document.getElementById("demo").innerHTML = juice.length;
  document.getElementById("demo1").innerHTML = juice;
  document.getElementById("demo2").innerHTML = water;
  </script>    
</body>
</body>
</html>

I want to do addition 4 to the above water variable and print the output, I tried coding as above, but I am unable to achieve the functionality .

What are you trying to achieve? juice is a one dimension array and in second for loop you are trying to access index juice[0][4], juice[1][4]. — Afshan Shujat
– Afshan Shujat, Commented Apr 25, 2016 at 8:55
@AfshanShujat: I want to do addition for the juice array elements with 4, for example: var a=[1,3,5,6,9,12,16, 18], I want to add 4 to that array elements, a[0]=1 by adding 4 to it, now a[0]=5 and my remaining output should be like a[5,7,9,10,13,16,20, 22] — Raja Shekar
– Raja Shekar, Commented Apr 25, 2016 at 9:20
You can do this by simply running each loop or for loop for array a and can add a[i] +=a[i]+4; — Afshan Shujat
– Afshan Shujat, Commented Apr 25, 2016 at 9:27
from this: today = mm+'/'+dd+'/'+yyyy+' $ '; juice[i] = today; looks like juice is an array of strings, or I'm missing something? — Giuseppe
– Giuseppe, Commented Apr 25, 2016 at 9:27
Do u want to add 4 more days to juice like juice = today+4 days? — Afshan Shujat
– Afshan Shujat, Commented Apr 25, 2016 at 9:32

Aditya Singh · Accepted Answer · 2016-04-25 09:46:31Z

2

Try using Array.prototype.reduce as below

var a = [1,3,5,6,9,12,16, 18];
var sum = a.reduce(function (prev, current) {
            return prev + current
          }, 0);

document.querySelector('#content').innerHTML = sum;

<div id='content'> </div>

If you need to add 4 to each use Array.prototype.map as below:

    var a = [1,3,5,6,9,12,16, 18];
    var sum = a.map(function (val) {
                return val + 4;
              });

    document.querySelector('#content').innerHTML = sum;

<div id='content'></div>

edited Apr 25, 2016 at 9:46

answered Apr 25, 2016 at 8:57

Aditya Singh

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1 Comment

Raja Shekar Over a year ago

thanks for your comment, but my requirement is different from your answer, I want to add 4 for each array element for example: var a=[1,3,5,6,9,12,16, 18], I want to add 4 to that array elements and my output should be like a[5,7,9,10,13,16,20, 22]

NateQ · Accepted Answer · 2016-04-25 09:16:30Z

The statement water[j] += juice[j][4]; in the second for loop won't work because you're using the += operator with array elements that have never been defined to begin with. For example if I have:

var x = [];
x[0] += 1;

this results in NaN. If I do it with strings like so:

var x = [];
x[0] += 'a';

it results in 'undefineda'. So that needs to be changed.

Secondly, what kind of integer addition you are trying to do? juice is an array of strings such as '04-24-2016' based on your code. Do you mean to add 4 days to whatever date is stored in juice[j]?

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